1. Field of the Invention
The present invention relates to a method and a system for extension of operating range of feedback in a ratio control in a continuously variable transmission.
2. Description of the Background Art
Continuously variable transmissions (CVT's) are transmissions that change a speed ratio continuously. This continuous nature of CVT's gives them an infinite number of speed ratios, making them very attractive for automotive use.
Various types of CVT are known. One such example is a CVT with pulley/V-belt power transfer. Another example is a CVT with disc/roller power transfer. The CVT of this type is often referred to as a toroidal-type CVT (TCVT) because it transmits torque from one rotating semi-toroidal disc to another semi-toroidal disc by traction rollers through a traction force. The two semi-toroidal discs form a toroidal cavity. In each toroidal cavity, it is preferred to have two traction rollers in equiangularly spaced relationship engaging the discs for transmission of motion therebetween. While three or four traction rollers may be disposed in spaced relationship in each toroidal cavity and will provide increased life for contact surfaces as the total surface area is increased, two traction rollers are preferred for simplicity.
Each traction roller is rotatably supported by a pivot trunnion, respectively. The pivot trunnions, in turn, are supported to pivot about their respective pivot axis. In order to controllably pivot the pivot trunnions for a ratio change, a hydraulic control means is provided. The hydraulic control means is included in a hydraulic cylinder at each pivot trunnion and includes a control volume defined in the hydraulic cylinder between a piston and an axial end of the hydraulic cylinder. The pistons within the hydraulic cylinders are connected to the pivot trunnions along their pivot axis by rods. The piston and its associated rod are thereby rotatable about the pivot axis with the associated pivot trunnion. Variation of the control volume causes the piston to move relative to the hydraulic cylinder, and applies a control force to displace the pivot trunnions. Control forces applied displace the pivot trunnions in the opposite directions along their pivot axis. As a result, the pivot trunnions are caused to pivot about their respective pivot axis, due to the forces present in the rotating toroidal discs, for initiating ratio change.
For terminating the ratio change when a desired ratio has been obtained, a feedback structure is provided. The feedback structure preferably includes a source of hydraulic pressure, and a ratio control valve for controlling the flow of hydraulic fluid for initiating ratio change. The feedback structure further includes a mechanism associated with at least one pivot trunnion to adjust the ratio control valve upon pivotal movement of the pivot trunnion to a desired ratio. The mechanism is preferably a cam connected to a pivot trunnion. The cam may be linked mechanically and/or electronically to operate the ratio control valve upon reaching a desired rotation.
It is known that there is a predetermined operational of the CVT where the CVT ratio has to be maintained at the largest ratio due to difficulty in deriving an actual value of the CVT ratio at very low vehicle speeds. JP-A 2000-283285 discloses a technique to cope with situation. According to this known technique, computation of the CVT ratio is suspended when the period or interval at which the computation is repeated exceeds the period of pulses of pulse train signals provided by input and output rotational speed sensors. In other words, the operating range of feedback in CVT ratio control has been extended until the period or interval at which the computation is repeated exceeds the period of pulses of pulse train signals provided by input and output rotational speed sensors.
A need remains for extending operating range of feedback in ratio control in a CVT for the purpose of reducing the structural margin needed accounting for various causes.